Clostridium difficile infection (CDI) is a significant cause of morbidity and mortality. Disease related to CDI ranges from asymptomatic colonization to mild diarrheal disease to severe pseudomembranous colitis. Despite the extensive characterization of the histologic lesions associated with C. difficile infection, virtually nothing is known mechanistically about the contribution of host factors in disease. It is likely that some of the clinical differences encountered in infected individuals are due to variation in host response to colonization with C. difficile. We hypothesize that variation in the host innate and adaptive responses for C. difficile are in part responsible for the different clinical phenotypes that are seen in the setting of CDI. Furthermore, we speculate that these variable responses reflect underlying genetic and possibly epigenetic differences in the host. To address these hypotheses we propose the following specific aims: In the first aim we will characterize the innate and adaptive immune response to C. difficile colonization and determine the role of this response in determining the disease manifestations. In the second aim we will identify serum fecal biomarkers and epigenetic changes that correlate distinct clinical outcomes. This aim will examine samples obtained from the Project area #1 for this purpose. The third aim will examine the interaction between host genetics/immune response and C. difficile genotype in the pathogenesis of disease. Mice with defined differences in host response will be challenged with C. difficile strains generated and characterized from the human studies in Project area #1. This will provide an examination ofthe relative contribution of host and pathogen factors in disease. These experiments will increase our understanding ofthe host and microbial mechanisms of C. difficile-induced colonic inflammation vs. asymptomatic colonization and identify biomarkers that can be used to improve clinical treatments and outcomes.